1. Field of the Invention
This invention relates generally to the field of golf equipment and relates more particularly to an improved golf club shaft with manually adjustable stiffness.
2. Description of the Related Art
Golfers of all skill levels are continually striving to increase the distance and accuracy of their golf shots. With the constant introduction of new materials and manufacturing methods to the sport of golf, equipment is playing an ever more important role in overall game improvement. Golf clubs, in particular, have benefited greatly from recent advances in technology. Larger club heads, titanium faces, and graphite shafts are just a few of the innovations that are enabling players to hit the ball further and straighter than ever before.
Widely regarded as the “engine” that drives the head of a golf club, a club's shaft has a substantial effect on a golfer's ability to achieve optimal contact between the club face and a golf ball. Undoubtedly, the most important characteristic of a shaft is its flexibility. A shaft's “flex rating” indicates the ease with which the shaft bends when forces are applied to it. The five most common flex ratings, in order from least flexible to most flexible, are Extra Stiff, Stiff, Regular, Senior, and Ladies (typically denoted by the letters X, S, R, A, and L, respectively). Generally, golfer's having higher swing speeds require a stiffer shaft, while golfer's having slower swing speeds fair better with a more flexible shaft. A shaft that is not properly matched to a particular golfer's swing can result in weaker ball contact as well as a variety of mishits.
To understand why the flexibility of golf club's shaft is so important, it is essential to first understand some of the forces that work on the shaft during a golf swing. As a player initiates his/her downswing, the head-end of the shaft will bend away from the ball in opposition to the forced movement of the club along the player's swing arc as a result of the F=ma force on the club head. By bending, the shaft stores energy in the manner of a spring. Immediately after the player achieves his/her peak swing acceleration, the energy stored in the shaft is released, forcing the shaft to “kick,” or unbend back toward a straight position. This “kicking” of the shaft increases the velocity of the club head along the swing arc as the bent portion of the shaft accelerates to return toward its equilibrium position.
In order to achieve ideal impact conditions, the club face should be traveling at its peak velocity, should neither be too shallow nor too lofted, and should be laterally square with respect to the target line. A shaft that is properly matched to a player's swing will kick at the correct time and will have returned to a substantially straight position at impact, indicating that the maximum amount of stored energy has been transferred from the shaft to the ball. If a player makes a good swing with a properly matched shaft, the ball will fly straight and far.
If a player uses a shaft that is too flexible for his/her particular swing, the shaft will kick prematurely and will flex past a straight position before contact, causing the shaft to be bent toward the ball at the time of impact. The result is a club face that is decelerating, overly shut, and excessively lofted when it meets the ball. Given an otherwise good swing by the player, the ball will tend to fly too high, to the left of the intended target (for a right-handed player), and short.
Conversely, if a player uses a shaft that is too stiff for his/her swing, the shaft will kick too late and will be bent away from the ball at the time of impact. The result is a club face that is open, shallow, and that has yet to reach its maximum potential velocity. Again, given an otherwise good swing by the player, the ball will tend to fly too low, to the right of the intended target (for a right-handed player), and short.
In addition to affecting the behavior of the shaft during a swing, shaft stiffness is also a major contributor to the overall feel of a golf club. When the clubface is brought into contact with a ball, a shaft that is more rigid will transmit a greater amount of vibration from the clubface to the golfer's hands. A more flexible shaft will dampen a greater amount of that vibration. More experienced players tend to prefer some measure of vibratory feedback to provide them with an indication of how the clubface interacts with the ball, such as whether they've struck the ball with the heel, the toe, or the center of the clubface. A stiffer shaft will provide such feedback. Less experienced players generally prefer a consistently solid feel through impact, regardless of how well they've actually struck the ball. A more flexible shaft will provide such feel.
Conventional golf club shafts are of fixed stiffness and, as described above, are generally only available in a limited number of incremental flex ratings. If a particular golfer is best matched with a shaft having a flex rating that falls somewhere between those that are commercially available, that golfer is forced to sacrifice some measure of performance and feel and must choose a shaft that is either too stiff or too flexible for his or her swing. Moreover, if a particular golfer's swing characteristics change over the course of time, that golfer must purchase a number of different shafts in order to maintain optimal matching.
It is therefore desirable to have a golf club shaft that can be manually adjusted to be more or less stiff. It is also desirable to have such a shaft that can be quickly and conveniently adjusted by a user or a technician.